1. Field of the Invention
The present invention relates to a method of manufacturing a silicon film having crystallinity or a film having crystallinity and containing silicon. The present invention disclosed in the present specification can be used, for example, in manufacturing a thin film transistor (called a TFT).
2. Description of the Related Art
A thin film transistor (hereinafter referred to a TFT, etc.) using a thin film semiconductor is known. This is constituted by forming a thin film semiconductor, especially, a silicon semiconductor film on a substrate, and using this thin film semiconductor. The TFT is used for various kinds of integrated circuit, and especially has attracted attention as a switching element provided for each pixel of an active matrix type liquid crystal display device, or as a driver element formed in a peripheral circuit portion. Moreover, the TFT has also attracted attention as an indispensable art for a multilayer structure integrated circuit (solid IC).
It is simple to use an amorphous silicon film as a silicon film used for the TFT. However, there is a problem that the electrical characteristics thereof is far lower than those of a single crystal semiconductor used for a semiconductor integrated circuit. Thus, the amorphous silicon film has been employed for only limited uses such as a switching element of an active matrix circuit. The characteristics of the TFT can be improved by using a silicon thin film having crystallinity.
A silicon film having crystallinity other than single crystal silicon is referred to as polycrystalline silicon, polysilicon, microcrystalline silicon, or the like. Such a silicon film having crystallinity can be obtained by first forming an amorphous silicon film, and then crystallizing the amorphous silicon film through heating (thermal annealing). This method is referred to as a solid phase growth method since the amorphous state is transformed into the crystalline state while the film maintains the solid state.
However, in the solid phase growth of silicon, heat temperature of 600.degree. C. or more, and time of 10 hours or more are required, so that there is a problem that it is difficult to use an inexpensive glass substrate as a substrate. For example, corning glass 7059 has a glass distortion point of 593.degree. C., so that there is a problem in carrying out thermal annealing at a temperature not lower than 600.degree. C. when consideration is given to enlarging an area of the substrate.
As to such a problem, according to the study by the present inventors, it has been proved that when a small amount of metal element of some kind, such as nickel and palladium, is deposited on the surface of an amorphous silicon film, and then heating is carried out, crystallization can be made under such conditions that a temperature is 550.degree. C. and a processing time is about 4 hours. Of course, when annealing is carried out at a temperature of 600.degree. C. for about 4 hours, a silicon film having more excellent crystallinity can be obtained (Japanese Patent Unexamined Publication No. Hei 6-244103).
The above-mentioned small amount of element (metal element for promoting crystallization) may be introduced by a method of depositing a coating film of the metal element or a compound thereof by a sputtering method (Japanese Patent Unexamined Publication No. Hei 6-244104), by a method of forming a coating film of the metal element or a compound thereof through a means such as a spin coating (Japanese Patent Unexamined Publication No. Hei 7-130652), by a method of forming a coating film by decomposing a gas containing the metal element through a means such as pyrolysis and plasma decomposition (Japanese Patent Unexamined Publication No. Hei 7-335548), or the like. Those methods may be changed according to the respective features.
Moreover, it is also possible to selectively adding the metal element into a specific portion and then to extend the crystal growth from the portion where the metal element has been added to the surrounding by heating (lateral growth method or side growth method). Since the crystalline silicon obtained by such a method has directionality in crystallization, the silicon shows extremely excellent properties in accordance with the directionality.
It is also effective to further improve the crystallinity by irradiation of intense light such as a laser beam after the crystallization step using the metal element (Japanese Patent Unexamined Publication No. Hei 7-307286). In the above-mentioned lateral growth method, it is also effective to carry out thermal oxidation subsequent to the lateral growth (Japanese Patent Unexamined Publication No. Hei 7-66425).
When crystallization was carried out using the metal element as described above, a more excellent crystalline silicon film was obtained under conditions of lower temperature and shorter time. Although temperature at heat treatment greatly depends on the kind of the amorphous silicon film, the temperature of 450 to 650.degree. C., especially 550 to 600.degree. C. was preferable.
However, the most serious problem of this method is the removal of the metal element. It can not be neglected for the metal element added into the silicon film to give bad influence to electrical characteristics and reliability. Especially, in the step of crystallization using the metal element, in the mechanism, since the metal element as mainly conductive silicide remains in a coating film, the metal element becomes a terrible cause for defects.
It is generally known that a metal element (especially, nickel, palladium, platinum, copper, silver, and gold) can be captured by a crystal defect, phosphorus, etc. For example, Japanese Patent Unexamined Publication No. Hei 8-330602 discloses a technique in which a phosphorus ion is implanted into a silicon film using a gate electrode as a mask, then the metal element contained in the silicon film is moved to a source and a drain region by carrying out thermal annealing (furnace annealing) or light annealing (laser annealing, etc.), and then the metal element is fixed (gettered) to reduce the concentration of the metal element in a channel formation region.
In Japanese Patent Unexamined Publication No. Hei 8-330602, when phosphorus is implanted into the source and drain regions, since a silicon film is made amorphous and crystal defects increase, the metal element can be gettered by phosphorus and the crystal defects. Here, phosphorus can be implanted not only into the source and drain regions but also into any portion as long as the portion is not a place where a channel formation region is to be provided. It is obvious for a skilled person that the metal element can be removed by the above method although the degree of removal is different according to the distance from the portion where phosphorus has been implanted.
In order to carry out gettering, it is necessary to carry out annealing for a sufficient time so that the metal element can move to a region where phosphorus has been implanted. Thus, thermal annealing is preferable for the purpose. However, annealing temperature effective for the gettering (although the temperature depends on the kind of the metal element) is generally more than 600.degree. C. When a process at such high temperature is carried out for a long time, the possibility of deforming a substrate is raised to cause the slippage of a mask (the misalignment of a mask) in a subsequent step of photolithography.
Thus, although the light annealing is preferable, Japanese Patent Unexamined Publication No. Hei 8-330602 does not particularly discuss a light source for the light annealing, and merely states that an excimer laser is used in an example. However, a pulse width of the excimer laser is not larger than 100 ns, and it is experimentally proved that gettering can not be sufficiently carried out by light irradiation for such a short time.
Japanese Patent Unexamined Publication No. Hei 8-330602 discloses that the substrate is irradiated with a laser beam from a place above the substrate. However, in any examples, since aluminum having high optical reflectivity is used and the thickness thereof is not less than 3,000 .ANG., it is difficult to give a sufficient amount of heat to a channel formation region so that the metal element is moved.